The invention is concerned with both the treating solution and process for applying a thin coating solution to an aluminum surface. It is particularly applicable for treating cleaned aluminum cans prior to the application of a decorative organic finish. The value of a good coating represents a substantial saving in cost and operating expense for it obviates the need to apply organic finish to the bottom of aluminum cans. However, the treating solution must be sufficiently satisfactory to improve the corrosion resistance of the can bottom.
In the past, satisfactory corrosion resistance and concomitant good adhesion of the organic finish were provided by typical metal treatment solutions composed of about 1% solution of a mixture of hexavalent chromium, phosphoric acid and fluoride. Greater sensitivity to preserving a clean, safe environment has caused industry to abandon these toxic compositions and to develop acceptable coatings which also provide adequate protection for the cans. There has been considerable investigative effort in this direction, with proper attention to extensive parameters of composition, concentration, pH, pretreatment, etc.
The compositions especially must meet the "pasteurization" test. Basically, the cans must be resistant to discoloration when subjected to moderately hot water for a half hour period of time. This is the industry-wide corrosion resistance test commonly referred to as "pasteurization." The test causes an uncoated or poorly coated aluminum surface to blacken or discolor in an unsightly and unacceptable fashion.
Coatings which pass the pasteurization test must additionally satisfy the tests to determine the quality of the adhesion of the subsequently applied organic coatings. Preferred coatings perform satisfactorily in both tests.
It is known that zirconium and fluoride compositions with boron provide satisfactory coatings. However, these compositions operate at a high pH, so that the coating solution bath must always be monitored for acid depletion and possible precipitation of the zirconium ion.
Tannin has been employed in coatings but has been demonstrated to be ineffective at pH values below 3. Tannin has been admixed variously with lithium, fluoride, and titanium ions.